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Fungicides used for in-season corn and soybean disease management are primarily in the chemical group of either strobilurin or triazole fungicides. Also, there are many pre-mix products that are available that contain both strobilurins and triazoles. Strobilurin and triazole fungicides are considered “locally systemic” which means they are absorbed into plant tissue.1 However, there is minimal systemic activity. For example, the fungicide may move slightly outward through a leaf from the point of uptake but does not move downward through the leaf stem, therefore new plant growth after application will not be protected.
Fungicides have preventative and/or curative activity. If a fungicide is on the plant before infection occurs it may act as a protective barrier preventing infection. Curative activity may occur when the fungicide is present within plant tissue and stops early growth of the pathogen in plant tissues.1 Fungicides with curative activity will not “cure” a plant from disease and are most effective if applied prior to infection or within the first 72 hours after infection.1
Effective fungicide performance requires good spray coverage. After application, fungicides degrade from the leaf surface and are generally considered effective for 21 days.2 Also, foliar-applied fungicides are ineffective in protecting plants from fungal pathogens that primarily enter through plant roots and cause diseases such as seedling diseases and crown rot in corn; also soybean diseases such as phytophthora root rot and sudden death syndrome.
Factors contributing to higher risk of disease for corn and soybean fields are similar and include the following: susceptible seed product, continuous corn or continuous soybean systems, history of disease infection in the field, current disease activity in the field, current and projected disease-favorable weather, high plant population/dense crop canopy, high yield potential, irrigation, and no-till or reduced tillage systems. Assessing individual fields for their risk of disease can serve as aid in identifying which fields should be prioritized for scouting.
High disease risk fields should be scouted prior to tasseling. It is important to scout fields in a continuous corn rotation and/or with greater than 35 percent residue and with a history of foliar disease. Many foliar diseases survive on corn residue and begin producing spores in wet weather. Moderate temperatures and humid weather conditions at vegetative growth stages and during grain fill can increase common rust and northern corn leaf blight in some fields. Warm and humid weather favors gray leaf spot, southern rust, and other fungal diseases that can be managed with fungicides. These diseases may develop to levels that will reduce yields if they substantially infect the ear leaf or leaves above the ear during the weeks of tasseling, pollination, and grain fill.
The ear leaf and leaves higher on corn plants should be protected from disease because they contribute the most energy supplied during grain fill.1 On susceptible or moderately susceptible seed products a fungicide application may be warranted if disease lesions are present on the third leaf below the ear or higher on 50 percent of the plants at tasseling.3 If a fungicide application is warranted, consider a VT (tasseling) to R1 (silking) application timing to maximize fungicide activity.
Fungicide Efficacy for Control of Corn Diseases, Purdue University, provides a list of fungicides, their efficacy against disease, recommended rates, and is available at https://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf
Keeping a close watch on fields for disease symptoms and a review of soybean growth stages can help properly time fungicide applications. Anthracnose, brown spot, Cercospora leaf blight, frogeye leaf spot, and soybean rust are diseases that may be managed with a timely fungicide treatment. Fungicide application in soybean differs from corn as there is more leaf development after application.
Application timing is essential for effective disease management. Fungicide applications made prior to the R1 (beginning flower) growth stage, or after the R6 (full seed) growth stage are often not economical.6 Research indicates a relatively early fungicide application at R2.5(full flower) growth stage may have yield benefit over fungicide applications at R4 (full pod) growth stage.4
Foliar diseases are often not an issue until the R3 (beginning pod) growth stage. If seed products lack disease resistance and conditions that favor disease development are anticipated at R3, a fungicide application may be beneficial. If left unchecked, potential yield loss from diseases may occur due to premature leaf drop reducing the photosynthesizing ability of the crop to produce grain. Weather conditions can also help determine proper fungicide timing in soybean. More than one fungicide application may be needed in environments with high disease pressure.
Additionally, a compatible herbicide or insecticide may be included if labeled for soybean to control weeds and insects that are present.
Timing for white mold or sclerotinia stem rot requires different timing as infection first occurs at R1 or the initiation of flowering. Therefore, an additional application may be necessary at a later growth stage to control other fungal pathogens.
Figure 1. Left - Soybean plant during beginning pod (R3) growth stage. Right - Soybean pods during the full pod (R4) growth stage. Photos used with permission from Iowa State University Extension and Outreach from the publication Soybean Growth and Development, PM 1945. Original publication date: 2004.
Sources: 1 Mueller, D. and Roberston. A. 2008. Preventative vs. curative fungicides. Iowa State University. Integrated Crop Management News.https://crops.extension.iastate.edu/cropnews/2008/07/preventative- vs-curative-fungicides
2 Hershman, D.E., Vincelli, P., and Kaiser, C.A. 2011. Foliar fungicide use in corn and soybean. University of Kentucky. PPFS-GEN-12. https://plantpathology.ca.uky.edu/
3 Robertson, A., Abendroth, L., and Elmore, E. 2007. Yield responsiveness of corn to foliar fungicide application in Iowa. https://crops.extension.iastate.edu/corn/production/foliarfungicide.html
4 Bohner, H. 2014. What is the correct time to apply foliar fungicides to soybeans? Crop Talk. Ontario Ministry of Agriculture, Food and Rural Affairs.http://www.omafra.gov.on.ca/english/crops/field/news/croptalk/2014/ct-0614a4.htm
5 Koger T. July 2, 2008. Soybeans reaching critical stage for fungicide. Delta Farm Press. Purdue University. Corn & Soybean FieldGuide. 2008 Edition.
6 Mueller, D., Robertson, A., and Pedersen, P. 2006. Asian soybean rust management strategies. PM2028. Iowa State University. https://crops.extension.iastate.edu/
7 Navi, S.S. 2014. Efficacy tests of foliar fungicides on soybean diseases and yield during 2012 and 2013 growing seasons in Northeast Iowa. Iowa State University.https://crops.extension.iastate.edu/cropnews/2014/02/efficacy-tests-foliar-fungicides-soybean-diseases-and-yield-during-2012-and-2013. Web sources verified 05/02/18. 140731064327